The Organic Rankine Cycle (ORC) is a method for operating a vapour-power process with a working medium other than water vapour. Used as the working medium are organic fluids with a low vaporisation temperature. The method is used particularly when the available temperature gradient between the heat source and the heat sink is too low for the operation of a turbine driven by water vapour. This is the case, for example, when producing electricity with the help of geothermal energy, in combined heat and power generation, and in the case of solar power plants and ocean thermal energy power plants. The Organic Rankine Cycle closely resembles the Clausius Rankine Cycle that is known from power plant technology and that is the standard cycle in the vapour power process. The organic material used as the working medium in the Organic Rankine Cycle allows a very good adjustment of the cycle to the temperature of the heat source. Depending on the temperature level, different working media are used: hydrocarbons, fluoridated (hydro)carbons (for example, tetrafluoroethane), aromatic hydrocarbons or silicone oils.
A pump brings the fluid working medium to the operating pressure while thereby providing relocation work and expansion or compression work. When the fluid working medium flows through a vaporiser, isothermal energy in the form of heat is fed to the vaporiser. Before the vaporiser, the working medium can be brought to boiling temperature in a pre-heater. After the vaporiser, the vapour of the working medium can be further superheated. Due to thermodynamic reasons, it is, however, possible to eliminate the space-intensive superheating, in which case only slight superheating by a few Kelvin, if any, is selected. The corresponding components pre-heater, vaporiser and superheater can be combined into a single heat-exchange component. In this heat exchanger, the heat that is fed in can be used for both pre-heating and vaporising, as well as for superheating, where applicable.
Used thereby as the heat source for the vaporising process is process heat or waste heat from upstream machines such as, e.g., a combustion engine. Due to the application of energy, the working medium vaporises completely or nearly completely. Saturated vapour forms at the outlet of the vaporiser. The working medium can also exit in a partially vaporised state. The exit as saturated vapour or as slightly superheated vapour is realistic.
The vapour of the working medium flows out of the vaporiser via a pressure pipe and to the ORC expander (for example, a screw expander or turbine) where this working medium is expanded polytropically to a lower pressure, as a result of which work is performed which is converted, e.g., by means of the turbine on its blades, into mechanical energy. In the ideal standard cycle, i.e., without losses and irreversibilities, the expansion would be isentropic. A generator coupled to the expander/turbine converts the mechanical energy into electrical energy. Alternatively to this, the mechanical energy can also be used directly, e.g., it can be (mechanically) coupled to the drive system of a vehicle, or the mechanical energy can be converted into hydraulic energy and fed into a hydraulic system.
Following this, the vapour then flows through a downstream condenser. There the vapour gives off the condensation heat and, where applicable, also sensible heat to a cooling circuit with a cooling media flow by means of desuperheating. The working medium condenses and passes into the liquid state of matter. Due to characteristic thermodynamic characteristics, the working medium usually exits the expansion machine in a superheated state and is consequently first desuperheated and then condensed in the condenser. Depending on the circuitry, a slight supercooling of the working medium can be achieved. The saturated fluid or slightly supercooled working medium can be temporarily stored, e.g., in a storage container. The pump then brings the saturated or (slightly) supercooled working medium from the storage container back to operating pressure, and consequently closes the cycle.